1. Field of the Invention
The present invention relates generally to the field of plant breeding and molecular biology. In particular, the invention relates to soybeans with increased α′ subunit of β-conglycinin content and materials for making such plants.
2. Description of Related Art
Soybeans are primarily grown for protein and oil. Soybean accounts for approximately 69% of the 161 million metric tons (MMT) of major protein meals in world trade (USDA, 2008). In the United States, about 30 MMT of soybean meal is consumed annually. Although soybeans produce a high quality cost-effective protein meal, there is a rising demand for increased nutritional value and functionality of the protein meal.
Composition and conformation are responsible for a protein's functionality. Compositional differences that could alter functionality include, for example, the ratio of protein fractions, variations in subunit concentrations within fractions, and differences in amino acid profiles. Soy proteins have four major water-extractable fractions (2S, 7S, 11S, and 15S) that can be isolated on the basis of their sedimentation coefficients. The 7S (β-conglycinin) and 11S (glycinin) proteins represent the majority of the fractions within the soybean.
Glycinin (11s globulin) is composed of five different subunits, designated A1aB2, A2B1a, A1bB1b, A5A4B3, A3B4, respectively. Each subunit is composed of two polypeptides, one acidic and one basic, covalently linked through a disulfide bond. The two polypeptide chains result from post-translational cleavage of proglycinin precursors; a step that occurs after the precursor enters the protein bodies (Chrispeels et al., 1982). Five major genes have been identified to encode these polypeptide subunits. They are designated as Gy1, Gy2, Gy3, Gy4 and Gy5, respectively (Nielsen et al., 1997). In addition, a pseudogene, gy6, and minor gene, Gy7, were also reported (Beilinson et al., 2002). Genetic mapping of these genes has been reported by various groups (Diers et al., 1993, Chen and Shoemaker 1998, Beilinson et al., 2002). Gy1 and Gy2 were located 3 kb apart and mapped to linkage group N (Nielsen et al., 1989), Gy3 was mapped to linkage group L (Beilinson et al., 2002). Gy4 and Gy5 were mapped to linkage groups O and F, respectively. In addition, B2G2 or “11S null” soybean variety has a unique seed composition including high level of β-conglycinin and low amount of glycinin. However, the B2G2 variety exhibits agronomically inferior characteristics such as low yield, excessive lodging and green seed. A number of breeding lines were developed, which carried all or parts of the mutations present in the B2G2 lines. Wu et al. provided breeding lines with agronomically acceptable characteristics (U.S. patent application Ser. No. 11/517,186).
β-conglycinin (7S), on the other hand, is composed of α (˜67 kda), α′(˜71 kDa) and β (˜50 kDa) subunits and each subunit is processed by co- and post-translational modifications (Ladin et al., 1987; Utsumi, 1992). Cgy2, 3 encode the α-subunit. Genetic analysis indicated that Cgy2 is tightly linked to Cgy3, whereas Cgy1 segregates independently of the other two. Cgy1 encodes the α′-subunit (Tsukada et al., 1986). The relative percentages of α′,α, and β chains in the trimer are ˜35, 45, and 20% of total β-conglycinin, respectively (Maruyama et al., 1999).
Soy protein functionality is partly dependent on the β-conglycinin-to-glycinin ratio and variations in the subunit compositions, which can vary among genotypes. The differences in composition and structure between β-conglycinin and glycinin are exhibited in both nutritional and functional properties. Glycinins contain more methionine and cysteine per unit than β-conglycinins, however soybeans lacking glycinins and enriched in β-conglycinins can have similar levels of total sulfur amino acids as soybeans containing glycinins. Glycinins are important for forming the protein particles that make up firm tofu gels (Tezuka, et al., 2000), but weaker gels are formed in the absence of β-conglycinin than those formed in the absence of glycinins (Tezuka, et al., 2004). The gelling properties of β-conglycinins and of soy protein isolates made from soybeans enriched in β-conglycinins show advantages under some conditions that may apply to meat applications (Nagano, et al., 1996; Rickert, et al., 2004). The gelling properties of β-conglycinin can be altered by varying the subunit composition with the alpha-subunit showing advantages (Salleh, 2004). The solubility and emulsifying properties of β-conglycinin are good in part because of the hydrophilic extension regions of the α and α′ subunits (Yarnauchi et al., 1991, Mauryama et al., 2002). There is potential to create valuable soybeans and ingredients for food use having increased β-conglycinin levels and decreased glycinin levels.
β-conglycinin has significant potential to positively impact human health (Baba et al., 2004). In particular, β-conglycinin has been found to lower cholesterol, triglycerides and visceral fat. Kohno et al. demonstrated that a significant reduction in triglycerol levels and viseral fat in human subjects that consumed 5 g of β-conglycinin per day (Kohno et al. 2006). Similarly, Nakamura et al. found that β-conglycinin upregulates genes associated with lipid metabolism in a primate model (2005). In addition, Nakamura et al. showed β-conglycinin had a significant effect preventing bone mineral density loss (2006). In addition, β-conglycinin demonstrated effects in lowering serum insulin and blood sugar (Moriyama et al. 2005). Due to β-conglycinin effects on triglycerides, cholesterol, fat, insulin and sugar levels, it may play an important role in health programs. In addition, β-conglycinin inhibits artery plaque formation in mice and may have similar affects in human subjects as well (Adams et al. 2004).
Furthermore, β-conglycinin may have a significant effect on intestinal microflora in humans. β-conglycinin is inhibits growth of harmful bacteria, such as E. coli, while stimulating growth of beneficial bacteria, such as bifidobacteria, in a number of animal models (Nakamura et al. 2004, Zou et al. 2005,). β-conglycinin could be used both to reduce E. coli growth after infection and maintain a healthy intestinal microbial community.
The α′ subunit of β-conglycinin may play a predominant role in many of the health benefits associated with β-conglycinin. A number of experiments using animal models have indicated that α′ subunit from soybean β-conglycinin could lower plasma triglycerides, and also increase LDL (“bad” cholesterol) removal from blood (Duranti et al., 2004, Moriyama et al., 2004, Adams et al., 2004, Nishi et al., 2003). Therefore, soybean varieties with an increased β-conglycinin content will have higher value than traditional varieties and will be suitable for use in nutrition drinks and other food products. In an attempt to identify the biologically active polypeptide(s), Manzoni et al. attempted to characterize biologically active polypeptides in β-conglycinin and indirectly demonstrated that the α′-subunit had a putative role in lowering cholesterol (Manzoni et al., 1998). Additionally, Manzoni et al. also demonstrated the influence of the α′ subunit on the increase in LDL uptake and degradation and LDL receptor mRNA levels (Manzoni et al., 2003). Duranti et al. (2004) demonstrated that the α′ subunit can lower triglycerides and plasma cholesterol in vivo.
The β-subunit of β-conglycinin has a number of health benefits as well. For instance, the β-subunit enhances satiety by causing cholecystokinin secretion (Takashi et al. 2003, Hara et al. 2004). Cholecystokinin is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein. Cholecystokinin, previously called is synthesized by I-cells and secreted in the duodenum, the first segment of the small intestine, and causes the release of digestive enzymes and bile from the pancreas and gallbladder, respectively. It also acts as a hunger suppressant. Hence, β-subunit may suppress appetite and may play a role in an overall weight management program.
The β-subunit may have a function in mental health as well. Soymorphin-5 are released by digesting the β-subunit with pancreatic elastase and leucine aminopeptidase. Soymorphin-5 is an opioid peptide. Opioids are chemical substances that have a morphine-like action in the body. Opioids are primarily used for pain relief. These agents work by binding to opioid receptors, which are found principally in the central nervous system and the gastrointestinal tract. Soymorphin-5 demonstrated anxiolytic effect after oral administration on mice, which suggest the intake of β-subunit may decrease mental stress (Agui et al. 2005).
Thus, the present invention produces soybeans with increased levels of the α′-subunit of β-conglycinin. Methods and compositions are disclosed herein to obtain soybeans with desirable protein composition.